The lighting industry is quietly going through a revolution where LED-based lighting will gradually replace incandescent and fluorescent devices. The main driver for this change is the efficiency of the light emitting device, with LED-based devices being as much as 10 time more efficient (lumens/Watt) than incandescent lamps. Currently the source LED is based on gallium nitride (GaN) which can emit either in the ultraviolet (UV) range or blue light in the visible (Vis) range depending on composition. The typical blue LED emitting region is comprised of InGaN quantum wells with GaN barriers. Adding more indium tunes the emission to the green, but the efficiency decreases. Rather than make white light by mixing efficient red and blue LEDs with an inefficient InGaN green LED, most white LEDs are made using red and green emitting phosphors excited by an efficient GaN blue LED. The phosphors emissions mix with some of the blue LED light to make white light.
The overall efficiency of the white LED depends not only on the LED and the phosphor materials, but on the light extraction efficiency of the package. One of the impediments to efficiency is backscatter from the phosphor, which is typically a powder with high refractive index. A popular choice is Ce:YAG (cerium doped yttrium aluminum garnet), which has a refractive index of 1.85 at 460 nm, the emission wavelength of a typical blue LED.                a. FIG. 1 is a drawing of a representative white light LED in a typical surface mount package showing the LED 10, wire bonds 12, phosphor particles 14 (illustrated as circular dots) in a silicone material 16 surrounding phosphor particles 14, a package substrate 18 and a package 20 for a Marubeni SMTW47 InGaN LED. The package 20 consists of the substrate 18, an epoxy resin lens 24 (illustrated as a black line), and a vessel or cup 22 made from white plastic or ceramic to contain the silicone-phosphor mixture, protect the LED, and to reflect the light from the package. What the illustration cannot show is that the blue emission from the LED is scattered in all directions by the high refractive index particles in the low refractive index silicone. The light is partly trapped in the phosphor-silicone mixture and is lost to residual absorption in the package.        
The phosphor powder is typically mixed with a silicone (refractive index 1.5) or epoxy and then applied to the top of the LED in its package as shown in FIG. 1. In the example illustrated by FIG. 1, a phosphor (14, red, or yellow) embedded in a silicone encapsulate 16 is excited by the 460 nm emission of the pn-junction of LED 10. However, there are undesirable properties when silicone encapsulation is used, among which are degradation over time and lack of thermal robustness; brittle conditions that arise from the use of the short light wavelengths; and undesirable backscatter 17 that arises from the refractive index mismatch between the phosphor (1.82-1.95) and the silicone (1.5). In view of these defects a better encapsulation host is desirable.
Some other details of the LED in its package are also shown in FIG. 1. The GaN LED is flip-chip bonded to a substrate, a design that provides good heat extraction and no shadowing by the bond pads or wire bonds. In addition the top surface has been roughened to prevent light trapping in the high refractive index GaN substrate. The red dashed line represents the InGaN quantum well emitting region.